In the past there have been several attempts to treat silica fillers to improve thereby the physical property profile of the elastomers and rubbers compounded therewith. It continues to be desirable to develop fillers that impart a high tear to the cure rubber while having a low viscosity in the uncured state and a high application rate or flow. While various cross-linkers in the room temperature vulcanizable silicone rubber formulation will produce a cured rubber having a lower cross-link density, modulation of the cross-link density does not appear to be the main factor contributing to high tear values. Rather, it appears that the main requirement to achieve a room temperature vulcanizable silicone rubber having both a high tear and a low viscosity is to choose the appropriate reinforcing filler. The most appropriate reinforcing fillers for room temperature vulcanizable silicone rubbers satisfying these requirements are the various forms of high surface area silica, particularly precipitated and fumed silica.
Fumed or precipitated silica as obtained from the manufacturers generally contain anywhere from 0.10 to 5.00 weight percent water or the equivalent of water in terms of surface hydroxyl groups. Using such hydrated silica fillers as fillers in heat curable or room temperature due to structuring of the filler within the rubber or elastomer matrix. Dehydrating the silica by means of a suitable calcination produced a powder that had a tendency to clump together, i.e. it was no longer a free flowing powder. Accordingly, such a calcined silica filler was very difficult to process. Additionally, special precautions were usually necessary to handle such a calcined silica filler because of the extremely hygroscopic nature of the calcined silica.
An improvement disclosed by Lucas in U.S. Pat. No. 2,938,009 consisted of treating the silica filer with a cyclic siloxane. During the treating process the cyclic siloxanes would chemically react with the surface hydroxyl groups of the silica. This served to prevent intercondensation between the hydroxyl groups of the silica filler thereby inhibiting structuring of the filler within the rubber or elastomer matrix. However, it was later determined that only a portion of the surface hydroxyl groups were inactivated by this method of treatment.
Subsequent treatments have evolved into the treatment of the silica filler with alkoxy-hydroxy silicones in combination with amines. Improvements over this art separated the simultaneous treatment of the silica filler with a first step involving treating the silica filler with ammonia or an amine followed by a second step treating the silica filler with a silicone derivative.
Further improvements in the various processes for treating silica fillers involve more complicated treating processes. One such improvement is represented by U.S. Pat. No. 3,635,743 where a reinforcing silica filler is first treated with ammonia and subsequently treated with hexamethyldisilazane. The treatment with hexamethyldisilazane is particularly effective in providing silica filled rubber compositions having a high Durometer and a high viscosity prior to vulcanization. The unvulcanized rubbers filled with a silica filler that is first treated with ammonia and then with hexamethyldisilazane are typically characterized by non-Newtonian flow properties.
The use of filled thermoplastic polymers for structural purposes such as styrene butadiene rubbers and the like has been advanced by the treatment of the mineral filler material with a very thin layer of certain organic compounds. As taught in U.S. Pat. No. 4,425,384 to Brownscombe the organic compounds comprising the surface treating agent are reactive at one end such that they bond in a covalent fashion with the surface of the filler and the other end of the surface treating molecule is such that by the nature of its similarity to the polymer being reinforced the polymer like end of the surface treating moiety interacts with the polymer being reinforced "as if the segment were part of the polymer." As taught in Brownscombe's '384 patent, the surface treating moiety or organic compound must have an "oxygen-reactive" end to chemically bond with the surface of the filler. While Brownscombe's '384 patent teaches the chemical bonding of the surface treating moiety to the surface of a mineral filler material, the other portions of the molecular structure of Brownscombe's surface treating agent are polymer-like such that they interact with the polymer being reinforced by the surface treated is filler.
In the case of silica fillers, there is no known test that will predict the tear and viscosity of rubbers compounded therewith. Since silicas initially possess a certain degree of hydration that varies from 4.8 SiOH to 12.0 SiOH groups per square nanometer, it is generally desirable to modify the effect of the surface silyl hydroxyl groups, (SiOH), by an appropriate chemical treatment. The surface hydroxyl groups render the silica hydrophilic. Hydrophilic silicas are good desiccants, hydrating easily. Chemically appropriate surface treatments may succeed in converting the hydrophilic silica surface to a hydrophobic surface. When the silica surface has been rendered hydrophobic, such a hydrophobicized silica generally imparts some type of improvement to the physical property profile of room temperature vulcanized (RTV) rubbers that incorporate such fillers (see for example, "Silicone Elastomer Developments," 1967-1977, Warick et al., Rubber Chemistry and Technology, Rubber Reviews, July-August 1979, vol. 52(3), ISSN 0035-9475).
Several means to measure the hydrophilic or hydrophobic nature of the surface of the silica filler exist, among them a water miscibility test, a solubility test utilizing a water methanol solvent blend, as well as various ignition loss tests to measure the amount organic matter coating the surface of the silica. While these tests estimate the degree of surface treatment they are not predictive of the physical property profile of the resultant rubber when the rubber is compounded with any given treated or untreated filler.
There are thus several different art methods currently known that teach the treating of siliceous fillers for various plastic, thermoplastic, elastomeric, and rubber applications. However, these art methods tend to be highly specific for the polymer formulation in question and vary, with an imperfect predictability, with the chemical nature of the filler comprising the filled polymer. Thus, methods of treating fumed silica fillers will tend to be different from those utilized to treat precipitated silica fillers because of the presence of a higher level of surface bound water in the case of the precipitated silicas as opposed to the almost anhydrous fumed silicas. The methods employed to chemically treat other fillers such as natural or synthetic calcium carbonate, finely divided minerals, finely divided bulk or porous metal oxides and the like will also tend to be specific to the surface chemistry of the material chosen.